1.Field of the Invention
The present invention relates to a base station and a method of monitoring channels, executed when a radio zone is switched to another, in a mobile communication system according to a time-division multiple access (TDMA), wherein a plurality of radio zones are formed by a plurality of base stations so that a plurality of mobile units communicate with each other.
1.Description of the Related Art
In recent years, mobile communications have been remarkably spread. In addition, similar to ordinary point-to-point communications, onrush of digitization has been pressing on in the world of mobile communications. The digital system can not only increase the amount of communications by leaps and bounds but also provide high quality communications and good protection against eavesdropping. In order to correspond to increasing communication demands under such background, the development of digital mobile communication systems has been actively progressed instead of conventional analog systems. Particularly, mobile communication systems according to the time-division multiple access are approaching to practical use. The time-division multiple access is an access method which multiplexes a plurality of communication paths on the time base. The time-division multiple access can simplify the system configuration and enhance system functions by utilizing idle time during communications. When the time-division multiple access is employed in combination with a multi-zone mode which divides a wide service area into a plurality of small radio zones for radio communications, a resulting communication system will achieve efficient utilization of frequencies and high cost effectiveness, so that such a system will be greatly expected as a mobile communication system. A conventional channel monitoring method, executed when switching a zone to another, in mobile communication systems according to the time-division multiple access, and a base station associated therewith will be explained below with reference to the accompanying drawings.
FIG. 5 is a block diagram showing the configuration of a mobile communication system according to the time-division multiple access. In FIG. 5, reference numeral 1 designates a mobile switching controller for controlling exchanges between an ordinary public network or other mobile communication systems and a radio channel within the mobile communication system itself and for managing travelling of mobile units, including registration of positions and so on, and radio communications of the mobile communication system; 2-5 base stations each for setting and resetting a radio channel, through which communications are performed with a mobile unit, under the management of the mobile switching controller 1 and for monitoring radio channels; 6-10 mobile units each for performing communications through the base stations 2-5 and the mobile switching controller 1 while moving within the system. The mobile unit may be a portable telephone, a portable terminal and so on, represented by a personal hand-held telephone. In this mobile communication system, radio zones 11-14 are defined for the base stations 2-5, respectively.
Next, FIG. 6 shows the configuration of a conventional base station. In FIG. 6, reference numeral 15 designates an antenna for transmitting and receiving radio signals from and to a mobile unit (not shown); 16 an RF unit for subjecting received radio signals to frequency conversion, amplification, and so on and outputting the processed signal to a subsequent modem 17, later described, or conversely for converting modulated signals inputted thereto from the modem 17 to radio signals; 17 the modem for demodulating modulated signals to produce baseband signals and conversely for generating modulated signals from baseband signals; 18 a frame processing unit for compressing and expanding baseband signals on the time base to generate and decompose time-division multiple access (TDMA) signals and for generating and decomposing frames; 19 a control channel controller for controlling the assignment of radio frequencies and transmission timing to a control channel; 20 a traffic channel controller for controlling the assignment of radio frequencies and slots and so on for traffic channels; 21 a channel monitor for monitoring channels, later described in detail; 22 an interface unit for interfacing data transmission between the mobile switching controller 1 and the base stations 2-5 shown in FIG. 5; and 23 a slot synchronization unit for extracting a slot synchronization signal from a signal from the mobile switching controller 1 received by the interface unit 22 and controlling a slot timing of the frame processing unit 18 in order to establish the synchronization among the base stations 2-5 at the timing of the extracted slot synchronization signal.
Explanation will be next given of the operation of the base station configured as shown in FIG. 6 when used as the base stations 2-5 shown in FIG. 5. Prior to starting communications, the base stations 2-5 each need to set a control channel. Upon turning on the power supply, the control channel controller 19 in each of the base stations 2-5 specifies a slot for a control channel for the frame processing unit 18 which has previously been determined, and transmits control data necessary for communications. The control data transmitted from the mobile switching controller 1 is converted by the frame processing unit 18 to data in a previously determined frame format, placed in the specified slot, and supplied to the modem 17. This signal is modulated by the modem 17, and supplied to the radio frequency (RF) unit 16 which conducts frequency-conversion and amplification on the modulated signal to produce a radio signal. The frequency of the radio signal thus produced is specified by the control channel controller 19. The radio signal is then sent from the antenna 15. The control channel, once starting the transmission, continues the transmission with the same slot.
The interface unit 22 receives and transmits communication data and control data from and to the mobile switching controller 1. Between the mobile switching controller 1 and the base stations 2-5, the control data is transmitted through a D-channel while the communication data through a B-channel. This data transmission will be explained with reference to a data format shown FIG. 9. It should be noted however that the data format of FIG. 9 is merely illustrative. The data transmission between the mobile switching controller 1 and the base stations 2-5 is performed in frame units. Each frame is composed of a header, a slot synchronization signal, the D-channel and the B-channel. The slot synchronization signal is used to establish the synchronization of the respective base stations 2-5 when data is transmitted from the mobile switching controller 1 to the base stations 2-5. The slot synchronization unit 23 extracts a change in the slot synchronization signal in the frame and generates a timing signal indicating the timing at which the frame processing unit 18 generates a TDMA signal. This enables slots assigned to the respective base stations 2-5 to be synchronized with each other.
Next, the configuration of the mobile units 6-10, communicating with the base stations 2-5, will be briefly explained, though not referring to drawings. Each of the mobile units 6-10 has an antenna; an RF unit; a modem; a frame processing unit; a control channel controller for controlling these components; and a traffic channel controller, similar to the base stations 2-5. When the mobile units 6-10 are portable telephones, they additionally comprise a microphone, a speaker and so on. The operations of the antenna, RF unit, frame processing unit, control channel controller, and traffic channel controller of each mobile unit are basically identical to those of the correspondents in the base stations 2-5. However, the base station differs from the mobile unit in that the former starts data transmission upon turning on the power supply, while the latter starts data reception, and in that the former has the right of determining a frequency and a slot to be used, while the latter is not allowed to determine these parameters.
The mobile units 6-10, when transmitting control data to the base stations 2-5, do so in conformity to the transmission timings of control channels from the respective base stations 2-5. More specifically, since control channels from the base stations 2-5 to the mobile units 6-10 and control channels from the mobile units 6-10 to the base stations 2-5 have transmission timings thereof which are defined according to the method such as the time-division multiple access (TDMA) or time-division multiple access time-division duplex (TDMA-TDD) or the like, the mobile units 6-10 can know their transmission timings of the control channels to the base stations 2-5 by receiving the control data from the base stations 2-5. In each of the base stations 2-5, the control data is received through the antenna 15, subjected to radio-frequency conversion in the RF unit 16, demodulated to a baseband signal by the modem 17, and inputted to the control channel controller 19 through the frame processing unit 18. Thus, the control channel controller 19 operates in accordance with the contents of the control data.
When the control channels have been set in this manner, the base stations 2-5 are enabled to communicate with the mobile units 6-10. The procedure of the communications will be explained below. First, for starting the communications, the traffic channel controller 20 in each of the base stations 2-5 specifies a slot for a traffic channel to the frame processing unit 18 and a frequency of radio signals to the RF unit 16. Assuming that the communications are made from the base stations 2-5 to the mobile units 6-10, communication data as shown in FIG. 9, transmitted from the mobile switching controller 1 through the B-channel, is extracted by the interface unit 22, and converted to a signal in a predetermined frame format by the frame processing unit 18. Then, the formatted signal is placed in a predetermined slot specified by the traffic channel controller 20 and inputted to the modem 17 in which the formatted signal is modulated. This modulated signal is converted to a radio signal at the frequency specified by the traffic channel controller 20 in the RF unit 16 and then transmitted through the antenna 15. The transmission in the reverse direction from the mobile units 6-10 to the base stations 2-5 is also performed basically in a similar manner. Specifically, communication data from each mobile unit 6-10 is transmitted after it has been processed to be a radio signal in the RF unit, and received by the antenna 15 of each base stations 2-5. This radio signal is subjected to frequency conversion and amplification to be a modulated signal in the RF unit 16 under the control of the traffic channel controller 20. The modulated signal is then supplied to the modem 17 and demodulated thereby to be a baseband signal. The baseband signal is decomposed by the frame processing unit 18 to produce a data signal which is transmitted to the mobile switching controller 1. The data signal is further formatted in the frame as shown in FIG. 9 by the interface unit 22 and inputted to the mobile switching controller 1.
Incidentally, when one of the mobile units 6-10 moves from a radio zone 11-14 to another radio zone 11-14, radio channel monitoring is performed. In this event, the mobile switching controller 1 first transmits a channel monitor request signal which is inputted to the traffic channel controller 20 through the interface unit 22. This channel monitor is performed in order to measure the level of a received radio signal at the time the channel monitor is requested, with specified radio frequency and slot. At this time, a slot and a radio frequency for monitoring a received radio signal are specified to the base stations 2-5 by the mobile switching controller 1. The traffic channel controller 20, upon receiving the channel monitor request signal, instructs the channel monitor 21 to monitor a channel with the specified slot and radio frequency. The channel monitor 21, after monitoring the channel, informs the traffic channel controller 20 of the measurement result. The traffic channel controller 20 further transmits this measurement result to the mobile switching controller 1 through the interface unit 22. Incidentally, if a specified slot for a channel to be monitored is being used when a channel monitor is requested, another slot must be specified for monitoring the channel in place of the previously specified slot. In this event, the mobile switching controller 1 instructs the base stations 2-5 to switch the previously specified slot to another. This instruction is inputted to the traffic channel controller 20 through the interface unit 22 in each of the respective base stations 2-5. The traffic channel controller 20 instructs the mobile units 6-10 under communication to switch the previously specified slot to another through a traffic channel and thereafter carries out the slot switching.
How to carry out this slot switching will be explained along a sequence of operations associated therewith, illustrated in FIG. 10. For the slot switching, the traffic channel controller 20 in each of the base stations 2-5 needs to control the channel monitor 21 to search for an empty channel (a radio frequency and a slot). When an empty channel has been found, the traffic channel controller 20 utilizes a traffic channel to inform the mobile units 6-10 of the channel to be switched which was found as a result of the search. The transmission of information indicative of the channel to be switched by use of the traffic channel is performed, in the case of audio communications, such that audio signals are not transmitted, for example, for one frame period, and the information indicative of the channel to be switched is transmitted instead of the audio signals. In this event, since only one frame of the audio signals is dropped, deterioration of the audio quality is not perceived by the mobile units 6-10. It is therefore possible to transmit channel switching information while maintaining a call. The base stations 2-5 and the mobile units 6-10, after transmitting the channel switching information, each transmit a synchronization burst as well as try to establish the synchronization with the synchronization burst from the opposite party. Here, the synchronization burst is a training radio signal for the modems 17 in the base stations 2-5 and for modems in the mobile units 6-10 to operate correctly, and is not used to carry information to be transmitted. The synchronization bursts are communicated by use of a radio frequency and a slot to be switched. After the synchronization has been established to enable radio transmission, the radio frequency and the slot used for transmitting the synchronization burst is again assigned to a normal traffic channel, thus completing the slot switching.
However, since each of the conventional base stations has the separate traffic channel controller 20 and control channel controller 19 operating independently of each other, they are not allowed to switch a slot for a control channel to a slot for a traffic channel, and troublesome switching operations must be performed for the slot switching.
For better understanding, referring to FIGS. 7 and 8, explanation is given of how traffic channel monitoring is troublesome, when performed by use of the conventional base stations configured as described above. FIG. 7 shows a sequence chart of the traffic channel monitoring in accordance with a prior art method, and FIG. 8 shows an example of channels used by the respective base stations 2-5. More specifically, shown in FIGS. 7 and 8 are frames transmitted from the mobile units 6-10 to the base stations 2-5, where the traffic channel is abbreviated as TCH and the control channel as CCH. For simplifying the explanation, it is assumed herein that the mobile unit 7 is moving from the radio zone 11 to the radio zone 14 as shown in FIG. 5, and explanation will be given of traffic channel monitoring requested by the mobile unit 7 in association with this zone switching.
It should be noted that while a four-division multiple access is illustrated in FIG. 8, the following explanation does not depend on the number of multiplexed channels and is applicable to the time-division multiple access time-division duplex (TDMA-TDD) method. This mode enables bi-directional communications of frames from the mobile units 6-10 to the base stations 2-5 and, conversely, frames from the base stations 2-5 to the mobile units 6-10, wherein data to be transmitted is treated as a single frame. A traffic channel used by the mobile unit 7 is designated "TCH7"in FIG. 8. It is also assumed that the mobile switching controller 1 has already detected that zone switching is required to the mobile unit 7.
Referring first Co FIG. 7, channel monitoring is mainly led by the mobile switching controller 1. Since the mobile switching controller 1 manages radio communications performed by the base stations 2-5, the mobile switching controller 1 is aware of which slot is being used by which of the base stations 2-5. For this reason, the mobile switching controller 1 first requests TCH monitoring (a) to the base station 3 which is not using the same slot as that for the traffic channel of the mobile unit 7. The base station 3, upon receiving the TCH monitoring request (a), monitors the traffic channel of the mobile unit 7 and transmits the monitoring result report (b) to the mobile switching controller 1.
The mobile switching controller 1 next sends a slot switching instruction (c) to the base station 2 to instruct the base station 2 to switch the slot for the traffic channel between the mobile unit 7 and the base station 2. The base station 2, upon receiving the slot switching instruction (c), sends a slot switching instruction (d) to the mobile unit 7. This slot switching is carried out by the previously explained procedure. Specifically, the synchronization burst is transmitted (e) and received (f) between the base station 2 and the mobile unit 7 before the slot switching is performed. When the slot switching is completed, the base station 2 sends information indicating that the slot switching has been completed (g) to the mobile switching controller 1.
Subsequently, the mobile switching controller 1 sends a TCH monitoring request (h) to the base station 4 which is not using the same slot as that for the new traffic channel of the mobile unit 7. The base station 4, upon receiving the TCH monitoring request (h), monitors the traffic channel of the mobile unit 7 (i), and sends a monitoring result report (j) to the mobile switching controller 1.
The mobile switching controller 1 next issues a slot switching instruction (k) to the base station 5 in order to enable the base station 5 to monitor the traffic channel of the mobile unit 7, and performs slot switching (1) such that the same slot as that for the traffic channel of the mobile unit 7 becomes empty. This switching is done in order to enable radio signals transmitted from the mobile unit 7 to be monitored by using an empty slot while the base station 5 continues a call. After the slot switching is completed, the base station 5 transmits information indicating that the slot switching has been completed (m) to the mobile switching controller 1. Afterward, the mobile switching controller 1 transmits a TCH monitoring request (n) to the base station 5. The base station 5, upon receiving the TCH monitoring request (n), monitors the traffic channel of the mobile unit 7 (o) and transmits a monitoring result report (p) to the mobile switching controller 1. Thus, a sequence of TCH monitoring operations are completed, with the result that the mobile switching controller 1 selects one of the base stations 2-5 which is in the best receiving condition to determine a radio zone to be switched.